As is known, the horizontal position of an electron beam on the screen of a cathode ray tube (CRT) is dependent on a horizontal drive signal provided by a deflection control integrated circuit (IC), which has generally been an analogue IC. The horizontal drive signal comprises a pulse having a period corresponding to the length of each line of the video signal to be displayed on the CRT. One edge of the horizontal drive signal pulse is used to control the electron beam to "flyback" from the end of one line on the CRT to the beginning of the next line by changing the direction of the current through the magnetic coils controlling the position of the electron beam. When this happens, the sudden change in direction of current through the coils causes a voltage "flyback" pulse to be generated. However, there is a delay between the timing of the edge of the horizontal drive signal pulse and the flyback pulse. This delay is variable depending on various factors, including the type of monitor and resolution being used and the ambient temperature and is not programmable.
A horizontal synchronisation signal is also provided having a horizontal synchronisation pulse for timing purposes. The frequency of the horizontal synchronisation signal is the same as the frequency of the horizontal drive signal, but the two signals are not necessarily in phase with each other, since the horizontal drive signal must be adjusted to compensate for the variability of the flyback delay and in order to adjust the horizontal position of the video signal displayed on the CRT screen.
The actual horizontal position of the video signal on the CRT screen is set by a horizontal position delay between the timing of an edge of the horizontal synchronisation pulse and the flyback pulse. However, since the timing of the flyback pulse is variably delayed with respect to the timing of the edge of the horizontal drive pulse, it is necessary to have some way of generating the horizontal drive signal such that the edge of the horizontal drive signal pulse is generated at the correct time to cause the flyback pulse to occur with the set horizontal position delay after the edge of the horizontal synchronisation pulse.